1. Field of the Invention:
This invention relates to a ball type safety valve used in a subterranean oil or gas well for automatically closing the well in response to a change in control fluid pressure.
2. Description of the Prior Art:
The most common types of downhole safety valve for automatically closing a subterranean fluid production conduit such as a tubing string extending within the casing of a producing oil or gas well are ball type or flapper type valves. Ball type valves employ a rotatable ball valve member which has a flow passage extending through the spherical ball element. The flow passage can be aligned with the tubing string to permit flow through the well. Rotation of the ball about a transverse axis through an angle of 90.degree. will close the tubing since valve seats and seals located above and below the valve will continuously circumferentially seal along a path on the exterior of the ball element. These ball type safety valves have a full opening and permit full flow through the tubing and through the valve itself.
Ball type safety valves may be used as either wireline safety valves in which a lock assembly is used to secure the safety valve in a nipple incorporated into the tubing string or as safety valves incorporated directly into the tubing string. In either case rotary actuation of the ball valve conventionally occurs as a result of axial movement of the ball valve element. A camming means normally comprising a separate pin engaging a milled slot in the exterior of the ball offset from the primary rotation pins is generally fixed relative to the moving ball. As the ball moves from a first to a second axial position the rotation pin moves relative to the cam pin and the ball is rotated from the closed position to the open position through an angle of 90.degree.. Conventional ball valve elements require movement of the ball away from the valve seats during rotation of the ball itself. This prevents damage to elastomeric seals positioned on the ball valve seats. Longitudinal movement of the ball valve element is generally imparted by longitudinal movement of a flow tube having a surface exposed to control fluid pressure. In general this flow tube will be spring-loaded such that a decrease in control fluid pressure acting on the flow tube will urge the flow tube upward thus permitting movement of the ball valve from the open to the closed position and automatically closing the tubing.
U.S. Pat. No. 4,289,205 shows one ball type safety valve in which the ball is positioned on a support member and positioned within an axially movable cage. During actuation of this ball the ball first moves away from the upper seat member to permit rotation. Rotation of the ball occurs upon axial movement of the ball support member and the ball relative to the cage. This rotation occurs due to the presence of an offset camming pin interconnecting the ball and the cage and permitting rotation about a transverse axis.